The title "Finding Dory" evokes a bunch of franchise-happy studio execs sitting around, looking at financial statements and wondering how to come up with a sequel given that Nemo is no longer missing.

The film's laboured opening scenes do little to dissipate this odour of a cash-in: we meet young Dory (Sloane Murray), being taught by her parents to say "I suffer from short-term memory loss". She's shamelessly, cynically cute; puppyish eyes, voice inflected with a gleeful gurgle. She’s shown primarily through sudden recollections by her older incarnation (Ellen DeGeneres), designed to communicate something of what it is like to have her condition.

But DeGeneres' role gradually becomes more structured and she's given a chance to show her voice-acting nous, putting in a shining, smiling performance, at once sad, funny and sensitive.

The plot is functionally equivalent to Nemo, only instead of a dentist’s fish-tank, Marlin and Nemo have to rescue Dory from a sort of aquatic hospital, presided over by a disembodied Sigourney Weaver. Here Dory meets Hank, a grouchy, seven-legged octopus, and perhaps one of Pixar’s greatest visual creations. Instead of dreaming of escape like everyone else, he wants to remain caged forever, and so performs a kind of reverse prison-break, Tarzan-swinging from pipes and, at one point, camouflaging himself as a cactus. As with the Parisian underbelly of Ratatouille, Pixar is at its most vital and brilliant when animating the minutiae of worlds ordinarily hidden to us.

A lot of the plotting is decorative, and the title serves as a neat, if obvious, double entendre: they are finding Dory, but she is also finding herself, discovering and overcoming the impositions of her condition. Her relationship with her parents is heart-breaking, and must be one of the most credible depictions of raising a child with special-needs recently committed to screen.

Not every character is treated so gently, however. A mangy, barely with-it seabird is made the perpetual butt of a joke that seems vaguely predicated on how funny it is when we can’t understand the mentally ill, while a gormless sea lion’s repeated exclusion from the other sea lions’ rock is another of the designated jokes. It feels slightly problematic that a film with such a present and emotive message about disability appears to find so much humour in the disabled.

Thankfully the moments where Finding Dory threatens to drown in a current of covert ableism are washed away as the central character's story comes to its inevitably happy conclusion. Fears of a cash-in are unfounded: this is a credible, gorgeous film in its own right.